1. Field of the Invention
The present invention relates to a thin film transistor, a semiconductor device, a display, a crystallization method, and a method of manufacturing the thin film transistor.
2. Description of the Related Art
Since a thin film transistor (hereinafter referred to as a TFT) has characteristics wherein the transistor can be formed on the surface of anything, it is possible to directly form a circuit for display on, for example, a display panel surface of a display. In the TFT of an initial stage of development, a channel region has been formed in an amorphous silicon layer deposited on a substrate. In the existing TFT, the channel region is formed in a polycrystalline silicon film formed by crystallizing the amorphous silicon layer. Accordingly, carrier mobility is about accelerated tenfold.
The polycrystalline silicon film has a structure in which micro single crystals having a grain size of about 0.1 μm are irregularly scattered. In the TFT in which the channel region is formed in this polycrystalline silicon film, a large number of crystal grain boundaries exist in the channel region owing to micro single crystals. The crystal grain boundaries indicate resistances to electrons or holes which move in the channel region. Since the number of the crystal grain boundaries differs betweens the adjacent TFTs, fluctuations are generated in characteristics of the TFTs. The TFT formed in the polycrystalline silicon film in this manner has a problem that the fluctuations are generated in the characteristics between the TFTs.
The present inventors have proposed a crystallization method for forming large-sized crystal grains, in which one or a plurality of TFTs can be formed in one single crystal grain for a purpose of obtaining a transistor having characteristics equal to those of a transistor formed on a silicon wafer (Surface Science Vol. 21, No. 5, P278 to 287 [2000]), and they have proceeded with development of a technology for industrializing the method. Since the TFT is formed in the single crystal grain, transistor characteristics are not adversely affected by the crystal grain boundary, and TFT characteristics are largely improved unlike a conventional TFT in which the crystal grain boundaries exist in the channel region.
Heretofore, a driving circuit of a display such as a liquid crystal display has been formed in an amorphous semiconductor film or a polycrystalline semiconductor film deposited on a glass substrate which is a display panel. With enlargement of an IT market, handled information is digitized, processing is accelerated, and a high image quality is also demanded in the display. To satisfy this demand, it is preferable to form, for example, the TFT which drives each pixel in a crystalline semiconductor. This increases a switching speed, and the high image quality is possible.
Furthermore, in addition to the pixel switching circuit, the display includes: a digital-to-analog converter which converts digital video data into an analog video signal; a signal processing circuit such as a gate array which processes the digital video data; a driving circuit such as a signal line driving circuit or a scanning line driving circuit and the like. There is a demand that these circuits be integrated on the same display panel substrate as that of the pixel switching circuit. To satisfy such demand, a TFT having a higher carrier mobility needs to be developed.